1. Field of the Invention
This invention relates to document shredding machines of the kind comprising a cutter assembly including two rows of cutter discs mounted for rotation in opposite directions about respective parallel axes, the discs of one row being arranged to enter gaps between the discs of the other row with little or no clearance so that the overlapping discs collectively act to cut material fed into the nip of the cutter assembly into narrow strips. Such machines are hereinafter referred to as being "of the kind specified".
2. Description of the Prior Art
It has been proposed to sever the long strips of material thus formed into short lengths by forming in the periphery of each disc a plurality of angularly spaced cutouts or notches so arranged that the notches in any pair of adjacent overlapping discs, one from each row, do not register with one another as the discs rotate. This effectively ensures that the strips are cut or torn into short lengths substantially corresponding to the spacing between successive notches in the periphery of the discs, the transverse edges which interrupt the peripheral cutting edges of the discs serving to make transverse incisions in the material.
However, in one prior proposal the notches on all the discs of each row are arranged in straight lines parallel to the axis of rotation. This arrangement has the disadvantage that the transverse edges which interrupt the cutting edges of the discs in either row all engage the material simultaneously and there is a sudden increase in the resistance presented to rotation of the discs. This is repeated regularly as each row of notches comes into contact with the material being cut and results in a noisy, jerky action and requires a driving motor of greater power than would be the case if the cutters could be made to operate smoothly.
In one attempt to overcome this problem, it has been proposed to arrange the notches in each row of discs generally on a helix so that in each row the notches of adjacent discs are somewhat offset circumferentially in the same sense in a regular manner from those of adjacent discs. Thus, the transverse edges previously mentioned contact the material progressively along the length of the row and the cutting load is evened out.
This solution may be satisfactory where the cutter discs are immovable axially, for example where each row of discs is formed from a single roller which is milled to define the individual discs. However, such constructions are very expensive to produce, especially where the discs are required to be particularly thin so as to cut the material into fine strips. In an alternative and less expensive type of arrangement, the cutter discs can be formed as individual discs mounted on a shaft with interfitting formations so that the disc is rotated by the shaft. Normally, the shaft is provided with a keyway and the discs are centrally apertured to fit on such shaft and provided with an inwardly projecting tang to enter the keyway, or the shaft may be formed with an axially extending rib, the aperture of each disc having a peripheral recess in which said rib engages or the shaft may be of non-round sections, the apertures of the discs being shaped correspondingly. This construction inevitably allows the discs some freedom for axial movement and it has been found that if such discs are formed with notches arranged generally helically, in use, the discs tend to move axially in one direction so as to become packed tightly together at one end of the shafts on which they are mounted and thus give rise to a substantial increase in frictional resistance to rotation.